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Railway signalling is a system used on railways to control traffic safely, for example, to prevent trains from colliding. Trains are uniquely susceptible to collision because, running on fixed rails, they are not capable of avoiding a collision by steering away, as can a road vehicle; furthermore, trains cannot decelerate rapidly, and are frequently operating at speeds where by the time the driver/engineer can see an obstacle, the train cannot stop in time to avoid colliding with it. This necessity was at the base of the establishment of strict guidelines for time keeping and railway chronometers in 1891 by the general time inspector Webb C. Ball of Cleveland, Ohio, USA and in 1889 by the UK parliament passing the Regulation of Railways Act 1889 - a series of requirements on matters such as the implementation of interlocked block signalling and other safety measures as a direct result of the Armagh rail disaster in that year.

In Australia, railway signalling is known as safeworking, perhaps due to the fact that not all safeworking systems involve signals.

Most forms of train control involve messages being passed from those in charge of the rail network or portions of it (e.g., a stationmaster) to the train crew; these are known as 'signals' and from this the topic of train control is known as 'signalling'.

Contents 1 Timetable operation 1.1 Timetable and train order 2 Blocks 2.1 Fixed blocks 2.2 Moving blocks 2.3 Entering and leaving manually-controlled blocks 2.4 Track circuits and automatic blocks 2.4.1 Track Circuits 2.4.2 Axle counters 2.5 Permissive blocks 2.6 History of block signalling 3 Interlocking Signals 3.1 Safety systems 3.2 Colour light signals 3.3 Operation 4 Modern signaling in the U.S. 4.1 Automatic Block Signals 4.2 Rule 251 4.3 Rule 261 4.4 Track Warrant Control 4.5 Direct Traffic Control 4.6 Form D Control 4.7 Centralized Traffic Control 4.8 Signaling enhancements 5 References 6 See also 7 External links

[edit] Timetable operation

The simplest form of operation, in terms of equipment at least, is operation according to a timetable. Everything is laid down in advance and every train crew knows the timetable. Trains can only operate in pre-arranged time periods, during which they have 'possession' of the track and no other train can operate.

When trains are operating in opposing directions on a single-line railroad, meets are scheduled, where each train must wait for the other at a point they can pass. Neither is permitted to move until the other has arrived.

The timetable system has several disadvantages. The first is that there is no positive confirmation that the track ahead is clear; only that it should be clear. This system does not allow for breakdowns and other such problems. The timetable is set up in such a way that there should be sufficient time between trains for the crew of a broken-down or delayed train to walk back up the line far enough to set up warning flags, flares and the explosive devices known as detonators or torpedoes (UK and US practice, respectively) which alert a train crew to a blocked track ahead.

The second problem is the timetable system's inflexibility; trains cannot be added or delayed; trains cannot be rescheduled.

The third is a corollary of the second; the timetable system is inefficient. To give a little flexibility, the timetable must give trains a broad swath of time to allow for some delay. Thus, the line is possessed by the train for much longer than is really necessary.

Nonetheless, this system permits operation on a vast scale, with no requirements for any kind of communication that travels faster than a train. Timetable operation was the normal mode of operation on American railroads in the early days.

[edit] Timetable and train order

With the advent of the telegraph, a more sophisticated system became possible because the telegraph provided the first system available where messages could be transmitted faster than the trains themselves. The telegraph allows the dissemination of alterations to the timetable, known as train orders. These override the timetable, allowing the cancellation, rescheduling and addition of trains, and almost anything else. Sufficient time must be given, however, so that all train crews can receive the changed orders.

Train crews generally receive the orders at the next station at which they stop, or sometimes orders are handed up to a locomotive 'on the run' via a long staff. Train orders allowed train dispatchers to set up meets at sidings, force a train to wait at a siding for a priority train to pass from behind, and to keep at least one block spacing between trains going the same direction.

In North American railway traffic control, a "Y" (yellow) train order gave crews information about track speed (in areas where tracks and bridges needed repair). These were called "slow orders" by train crews. An "R" (red) order dealt with meets, waits and other important traffic control issuse. Orders were flagged at train stations by telegraph operators who signaled using a fixed "order board".

Timetable and train order operation was commonly used on American railroads until the 1960s, including some quite large operations such as the Wabash Railroad and the Nickel Plate Road. Train order traffic control was used in Canada until the late 1980s on the Algoma Central Railway and some spurs of the Canadian Pacific Railway.

Timetable and train order was not used widely outside North America and has been phased out in favor of radio dispatching on many light-traffic lines and electronic signals on higher-traffic lines. More detail on North American operating methods is available below.

[edit] Blocks

If two trains cannot be running on the same section of track at the same time, then they cannot collide. So railway lines are divided into sections known as blocks, and only one train is normally allowed in each block at one time. This principle forms the basis of most railway safety systems.

[edit] Fixed blocks

Most blocks are 'fixed' blocks, i.e. they delineate a section of track between two defined points. On timetable, train order, and token-based systems, blocks usually start and end at selected stations. On signalling-based systems, blocks start and end at signals.

The lengths of blocks are designed to allow trains to operate as frequently as necessary. A lightly-used branch line might have blocks many kilometres long, whilst a busy commuter railway might have blocks a few hundred metres long.

A train is not permitted to enter a block until a signal indicates that the train may proceed, a dispatcher or signalman instructs the driver accordingly, or the driver takes possession of the applicable token. In most cases, a train cannot enter the block until not only the block itself is clear of trains, but there is also an empty section beyond the end of the block for at least the distance required to stop the train. In signalling-based systems with closely-space signals, this overlap could be up to the signal following the one at the end of the section, effectively enforcing a space between trains of two blocks.

Historically some lines operated rules where certain large high speed trains were signalled under differing rules and only given the right of way if two blocks in front of the train were clear.

[edit] Moving blocks

A disadvantage of fixed blocks is that the faster trains are permitted to run, the longer the stopping distance, and therefore the longer the blocks need to be. This decreases a line's capacity.

With Moving Block, computers are used to calculate a 'safe zone', behind each moving train, which no other train may enter. The system depends on precise knowledge of where each train is and how fast it is moving. With Moving Block, lineside signals are unnecessary, and instructions are passed direct to the trains. It has the advantage of increasing track capacity by allowing trains to run much closer together.

Moving Block is in use on London's Docklands Light Railway and planned for future use on the Jubilee Line. It was supposed to be the enabling technology on the modernisation of Britain's West Coast Main Line which would allow the running of trains at a far higher maximum speed (140mph), but the technology was deemed not mature enough, considering the large number of junctions on the WCML, and the plan was dropped at much expense to the British taxpayer^{[citation needed]}. It forms part of the European Rail Traffic Management System's level-3 specification for future installation in the European Train Control System, which will (at level 3) feature moving blocks that allow trains to follow each other at exact braking distance.

[edit] Entering and leaving manually-controlled blocks

Before allowing a train to enter a block, the signalman must be certain that the block is not already occupied. And when a train leaves a block, he must communicate that fact to the signalman controlling entry to the block. Even if the signalman receives advice that the previous train has left the block, he is usually required to seek permission from the next signalbox to admit the next train into the section. When a train arrives at the end of the block section, before the signalman sends the message that the train has arrived, he must sight an end-of-train marker on the back of the last vehicle. This ensures that no part of the train has become detached and left in the section. The end of train marker might be a white disc by day or a steady or flashing red lamp. If the train has entered the next block before the signalman sees that the disc or lamp is missing, he will contact the next signalbox to stop the train and investigate.

[edit] Track circuits and automatic blocks

With automatic block signalling, signals indicate whether or not a train may enter the block based on automatic detection of whether the block is occupied or clear. The signals could also be controlled by a signalman, so that they only provide a proceed indication if both the signalman sets the signal accordingly and the block is clear.

[edit] Track Circuits

Main article: Track circuit

One of the most common methods of determining if a block is occupied or clear is by using a track circuit to detect the presence of a train in the block. The track at either end of the block is electrically insulated, and electrical current from a battery or power supply is fed to both running rails at one end of the block. A relay at the other end is connected to both rails. When the block is unoccupied, the circuit is completed, and the relay is energized. However, when a train passes a signal and enters a block, the metal wheels and axle of the train short-circuit the current in the block, and the relay is de-energized.

This method does not explicitly need to check that the entire train has left the block. If part of the train is left in the block, that part will continue to be detected by the track circuit.

This type of circuit is used to detect trains, both for the purpose of setting the signal indication and for providing various interlocking functions — for example, not permitting points to be moved when a train is standing on them. Electrical circuits are also used to prove switch points as being in proper position before a signal over them may be cleared. Modern UK trains carry electrical cables so that, in the event of a derailment fouling an adjacent track, the derailed train's guard can short-circuit the two rails of the adjacent track; this triggers danger signals on that track and can be used to prevent a collision with the derailed trains before the train's crew is able to contact the signalman.

When calculating the size of the blocks and hence the spacing between the signals, the following has to be taken into account:

• Line speed (the maximum speed the train is allowed to travel)
• Gradient (to compensate for the assistance or otherwise afforded to deceleration)
• The braking characteristics of the train(s) that travel on that line
• Sighting (the ability of the driver to see the signal)
• Reaction time (of the driver)

[edit] Axle counters

Main article: Axle counter

An alternative method of determining the occupied status of a block is with devices at the beginning and end of the block that count the number of wheels, axles, or etc. that enter the block and that leave the block, and compare the results. If the same number leave as enter, the block must be clear.

[edit] Permissive blocks

In a permissive block system trains are permitted to pass signals indicating the line ahead is occupied, but only to do so in a manner where they can stop safely driving by sight. This allows improved efficiency in some situations and is mostly used in the USA, and in most countries is restricted to freight trains only, and may be restricted depending on the level of visibility.

Permissive block working may also be used in an emergency, either when a driver is unable to get in touch with the signalman after being held at a danger signal for a specific time, although this is only allowed when the signal does not protect any conflicting moves, and also where the signaller is unable to get in touch with the next signalbox to make sure the previous train has passed, for example if the telegraph wires are down. In these cases, trains must proceed at very low speed (typically 20mph or less) and able to stop short of any obstruction, and in most cases it will not be allowed during times of poor visibility (e.g fog or falling snow).

An absolute block system is itself not entirely absolute. Multiple trains may enter a block given specific authorisation. This is necessary in order to join trains together, split trains, rescue failed trains and the like. The signalman in giving authorization also ensures the driver knows precisely what to expect ahead, and the driver must operate the train in a safe manner considering this information. Generally, the signal will remain at danger, and the driver will be given verbal authority, usually accompanied by a yellow flag, to pass the signal at danger, and the presence of the train in front will be explained. At locations where trains regularly enter occupied blocks, such as stations where coupling up takes place, a second, smaller signal arm known as a "calling on" signal will be provided for these movements; otherwise they are accomplished through train orders.

[edit] History of block signalling

In the very early days of railways, on double-tracked railway lines, where trains travelled in one direction on the same stretch of track, a means was needed to space out the trains to ensure that they did not collide. In the very early days of railways, men (originally called 'policemen') were employed to stand next to the line at certain intervals with a stop watch, these men used hand signals to signal to train drivers that a preceding train had passed more or less than a certain number of minutes ago, this was called "time interval working". If a train had passed the man only a short while ago, the following train was expected to slow down or stop to allow sufficient space to develop between the trains, to prevent a collision.

This system was flawed, however, as the watchman had no way of knowing whether the preceding train had cleared the tracks ahead. And so if the preceding train broke down or stopped for some reason, the following train would have no way of knowing, and collide with it rear-on. Accidents of this type were common in the early days of railways. However, with the invention of the electrical telegraph, it became possible for the station or signal box ahead to send a message (usually a bell ring) back to confirm that a train had passed and that the line ahead was clear; this was called the "block system".

Mechanical semaphore signals replaced hand signals in the early 1840s. When the all-clear message was received, a signalman in a signal box would pull a lever which would move the signal into the all-clear position. This required the placing of signal boxes at regular intervals along the line.

The block system came into use gradually during the 1850s and 1860s but became mandatory in the United Kingdom after Parliament passed legislation in 1889 as a response to numerous railway accidents, particularly Armagh. This required block signalling for passenger railways, along with interlocking and most of the practices still required and used today. Similar legislation was passed by the United States around the same period.

Not all blocks are signalled using fixed signals placed along the track. On single line railways in the UK, particularly those with low usage, it is common to use token systems that rely upon the physical possession of a unique object by a train's driver as authority to occupy the single line.

[edit] Interlocking Signals

Timetable and train order operation still has some significant flaws, such as an over-reliance on the ability of the crew of a stranded train to let other trains know of the problem, and a general intolerance for human error. When everything goes perfectly it works well, but mistakes are easy and deadly.

Timetable and train order is only suitable for railway lines which carry relatively little traffic, and is unworkable on busy rail lines because it requires great separation between trains. Where this is the case, physical signals need to be used (either mechanical semaphore signals, or - more commonly in the modern era - electric light signals) to show the train crew whether the line ahead is occupied and to ensure that sufficient space is kept between trains to allow them to stop.

[edit] Safety systems

The purpose of signalling is to inform the driver when it is safe to proceed on the line ahead. In early days the signalman was responsible for ensuring any points (US: switches) were set correctly before allowing a train to proceed. Mistakes were made and accidents occurred, sometimes with fatalities. The concept of interlocking of points, signals, and other appliances was introduced to improve safety. Interlocking prevents the signalman from operating appliances in an unsafe sequence, such as setting the signal to clear while one or more points in the route the signal governs are improperly set. Early interlocking systems used mechanical devices both to operate the signalling appliances and ensure their safe operation, but contemporary interlocking systems perform using complex electronic circuitry.

A second area of safety concern was fog. Because of the propensity for heavy fog in some parts of the British Isles, fog signal rules were established on the UK railway system to keep train traffic moving without incurring the severe delays that would be necessary if drivers had to stop or travel slowly up to each signal and read its indication. During heavy fog, fogsignalmen would be stationed at distant signals with a lantern and detonators — small explosive charges that could be strapped to the rail to be exploded by the wheels of a train. The fogsignalman's duty was to repeat the indication of the signal using his lantern; the semaphore blade was usually obscured by fog and hence invisible to the driver of a moving train. If the distant signal were displaying Caution (warning that the home signal was at danger), the detonators remained on the rail and the fogsignalman would show a yellow lamp to show Caution; if the distant signal were clear, the detonators would be removed from the rails and a green lamp would be displayed.

In the UK all trains were required to carry detonators so that in the event of an incident (such as an emergency brake application) the train crew could place them on the line to warn any other train approaching from the rear or on any adjacent tracks.

Britain's Great Western Railway introduced the Automatic Train Control (ATC) system in 1906. This system is the forerunner of today's Automatic Warning System (AWS) and consists of an electrical system that sounded a bell in the cab as the train approached a signal at clear. Power was fed through a metal ramp to a pickup on the underside of the locomotive to power the bell. An absence of the electrical voltage on the ramp caused a warning horn to sound in the locomotive's cab. The driver then had a set time to acknowledge the warning and start braking his train accordingly. If the driver did not acknowledge the warning, the brakes would be applied automatically. Where this was implemented, it did away with the need for fog signalling, since the driver could tell the state of the distant signal regardless of his ability to see it.

The current system of AWS in use on Britain's railways is similar in principle to the Great Western's ATC but does not rely on physical contact between a ramp and the train; instead an inductive system is used. The most recent train control systems use modern electronic systems to indicate the state of the signals ahead to the driver on cab displays at all times and can halt a train automatically if a signal is passed at danger.

The railroads of the United States use systems of different designs; some systems are similar in behavior to AWS wherein they provide warnings only in conjunction with fixed signal locations. Other systems provide a continuous in-cab signal indication.

[edit] Colour light signals

Main article: Railway signal

On most modern railways, detection of the train's position on the line and signal changing are done automatically, and colour-light signals have largely replaced mechanical ones.

Although signals vary widely between countries and even railways within a given country, a general guide to the lights displayed in a typical signal system would be:

• Green: Proceed at line speed. Expect to find next signal green or yellow.
• Yellow: Prepare for next signal to be at red.
• Red: Stop.

In the UK a variation of this is used whereby each set of signal lights has four lights in order from top down: yellow, green, yellow, red. The red and green signals are used as described, as is the lower yellow light. The upper yellow light is used to provide a 'double yellow' signal which serves as a warning that the next signal is at yellow, thus providing a warning of a red signal a further block in advance.

Double signalling is sometimes used; this is the method used in some areas of New South Wales, Australia. It derives from semaphore signalling. Two sets of lights are displayed, one above the other. The upper light is the condition at the current signal, and the lower is an indication of the aspect being shown the next signal. Some lights have a small lamp at the bottom; this is an indication to continue at low speed.

The double signals indicate:

• green over green: continue
• green over yellow: attention, next signal at green over red
• green over red: caution, next signal at stop
• red over red: stop and stay stopped
• red over red with small lamp lit: low speed, 25 km/h.

[edit] Operation

When a track in a signal block is occupied, the signal which the train has just passed automatically turns from green (or yellow) to red, the signal behind that one automatically turns yellow to warn following trains of the red signal, and the signals behind that one can show green.

If any train is following behind, the yellow signal will warn it to slow down in order to stop at the next signal. If, however, the train in front has passed into the next block, the following train will come across another yellow signal. If the train in front is travelling faster than the following train and clears two blocks, the following train will come across a green signal.

The genius of this relay-based track circuit system is its fail-safe mode of operation. If the relay accidentally becomes de-energized, it fails, or the circuit is otherwise disturbed, the track circuit's status and the potential for presence of a train are unknown. In all of these cases, the track circuit causes the associated signal to drop to red.

The track circuit is also used by signalmen to detect exactly where a train is on a line. In a signal box, a map of the stretch of track the signal box is controlling is usually put up on the wall (called a mimic panel), and when a train enters a particular block, a light representing that block on the map lights up to show the train's location. In the UK, in large signalboxes it is usually numbered with the train's headcode so that the signalman can see which train it is. Track circuits are also used to trigger automatic barriers (gates) on level crossings (grade crossings in U.S. usage).

[edit] Modern signaling in the U.S.

U.S. railroads have historically used a far greater variety of signaling systems than other countries. There have never been national standards for signal appearance and operation, so each of the hundreds of rail lines developed its own signaling techniques.

As Trains magazine describes:

This was no problem as long as crews stayed on home territory. But as roads merged, split, and spun off new short lines, and tenant operators such as Amtrak and regional commuter systems came into existence, train crews could find themselves on several different properties in the course of a work week.

The creation of Conrail in 1976 out of the remains of a half-dozen bankrupt railroads only made things worse. It was hard enough to rationalize the systems of the constituent companies, let alone interact with other operators over the dense Northeastern U.S. rail network.

After several years, the situation had become intolerable. Training costs were getting out of hand, because crews had to qualify separately on each road over which they might operate. Having to consult half a dozen rulebooks increased enormously the potential for a disastrous mistake.<ref>"NORAC: Northeast Operating Rules Advisory Committee: A common rulebook for a diverse Northeast" by Martin Graetz, Trains May 1, 2006, retrieved August 16, 2006</ref>

As railroad companies eventually began to standardize their rule books via industry-wide committees, the implementation of signal systems between railroads became, if not standardized, at least more similar. Different legacy systems still in use, however, mean that some signal indications can be shown in several different ways.

While each company is free to modify its operating practices (subject to the Code of Federal Regulations), there are three major groups of railroads that share rule books, and therefore, have similar operating practices. Major railroads on the East Coast have adopted the Northeast Operating Rules Advisory Committee (NORAC) rules. Most railroads west of the Mississippi River, as well as the Canadian Pacific Railway, use the General Code of Operating Rules (GCOR). All other Canadian railroads, including Canadian National, use the Canadian Rail Operating Rules (CROR). (Some railroads, including CSX and Norfolk Southern, still use proprietary rule books.) These rule books specify various methods of operation in both signaled territory and "dark territory," where manual methods of granting track authority must be used.

[edit] Automatic Block Signals

Automatic Block Signal, or ABS, systems consist of a series of signals that govern blocks of track between the signals. The signals are automatically activated by the conditions of the block beyond the signal. If a train is currently occupying a block, that block's signal will not allow a train in the previous block to proceed into the block, or will only allow it to proceed at a speed which allows the train to stop before colliding with the train or another object (also known as restricted speed).

Automatic block signals also detect the status of a following signal. If a signal is displaying a stop indication, the preceding signal will display an aspect that warns the train crew that the following signal may require the train to stop.

ABS systems detect track occupancy by passing a low-voltage current through the track between the signals and detecting whether the circuit is closed, open, or shorted. A train's metal wheels and axles will pass current from one rail to the other, thereby shorting the circuit. If the ABS system detects that the circuit is shorted between two signals, it understands that a train is occupying that block and will "drop" the signals (display a stop indication) on either side of that block to prevent another train from entering. ABS system electronics are also able to detect breaks in the rail or improperly-lined switches, which result in an open circuit. These will also cause the signal's aspect to drop, preventing any trains from entering the block and running the risk of bending, breaking, or overturning the rail and derailing or running through an improperly-lined switch.

[edit] Rule 251

NORAC rule 251 governs ABS on a track to be used in a single direction called the "current of traffic". Before the introduction of CTC, there were usually two or more tracks to allow for travel in both directions. The signals face the traffic, and moves against the current of traffic require a Form D. On a three-track railroad, typically the outer two tracks will be Rule 251 for opposite directions, and the middle track is Rule 261 to allow movements in both directions. On a four-track railroad, one or both the middle tracks are typically Rule 261 and the outer tracks are Rule 251.

Like ABS signals, these signals are not controlled directly by the train dispatcher but are instead automatically activated by track occupancy. Rule 251 is mostly used in the dense northeastern United States.

[edit] Rule 261

Rule 261 allows for a single track to be operated in both directions by setting a "current of traffic" for the track. Signals that are for the current of traffic will normally show Proceed, while those against the current of traffic will show Stop Signal. Typically, a traffic lever in an interlocking tower controls the current of traffic.

[edit] Track Warrant Control

In Track Warrant Control, or TWC, the train dispatcher issues "track warrants" via radio that authorize the train between two specified limits. The limits are often mileposts or stations. The track warrant may authorize a train to proceed to a station and "clear the main," or enter a siding so an oncoming train can pass. Generally, no more than one train or piece of equipment may be given the same or overlapping limits of authority, unless the movements will be made at restricted speed. While TWC is generally used in dark territory, it may be combined with ABS to enable more than one train to be given the same authority (although this generally only applies to trains moving in the same direction). This reduces the workload of the dispatcher and train crew, as new track warrants do not need to be copied every few minutes to ensure that following trains are not delayed due to running out of authority.

[edit] Direct Traffic Control

Direct Traffic Control, or DTC, is similar to TWC, except that the rail line is divided up into predefined blocks--somewhat similar to ABS blocks without the signals--and dispatchers authorize trains to proceed in a specified number of blocks. Only one train may occupy a stretch of authority (which may consist of a single block or a stretch of dozens) at any given time, unless movements are to be made at restricted speed. Like TWC, DTC may be combined with ABS in high-traffic areas to aid with train separation and safety.

[edit] Form D Control

Form D Control System, or DCS, is a system similar to Track Warrant Control that is used by railroads subscribing to NORAC (Track Warrant Control is a GCOR term). The name comes from the form that train crews copy the authority on. A sample Form D is available here; line two is used to grant authority for occupying the track.

[edit] Centralized Traffic Control

Many large or heavily-trafficked railways use Centralized Traffic Control, or CTC. In CTC, train dispatchers (called "control operators" on larger railroads) monitor the location of trains electronically using track circuits, direct traffic by controlling the indication of absolute signals, and efficiently arrange oncoming train meets by automatically lining electronically-operated ("dual controlled") switches to cause trains to enter sidings. In addition to absolute signals controlled by the dispatcher, CTC territory usually includes intermediate signals between the absolute signals that operate similarly to ABS signals. These signals help keep trains separated between absolute signals, which can often be dozens of miles apart.

Because of the electronics used (the signals, the electronic switches, and the communications infrastructure), CTC track is much more expensive to build and maintain than track used in other methods of operation. In other methods of operation, train crews must stop to line switches when entering a siding (and may often be required to line them back when the entire train has left the main track, causing members of the train crew to walk long distances, unless otherwise authorized by the train dispatcher). CTC also enhances safety, as it allows train dispatchers to monitor the movement of trains to ensure they do not overrun their authority and provides for detecting the condition of the track.

In the western United States, railroads have built large sections of multiple main track, but they are operated under CTC instead of Rule 251. This allows the dispatcher greater flexibility in managing traffic by having some trains "cross over" to the other main track to pass slower or stopped trains or allow access by trains moving in both directions to spurs and industries on both sides of the track. In some extremely high-density corridors, there may be three or even more CTC-controlled tracks in parallel.

[edit] Signaling enhancements

Cab signalling can be used as an overlay for ABS, Rule 251 and CTC. In some areas, it has replaced wayside signals. This system repeats in the train cab the signal aspect that was shown on the wayside signal at the entrance to the block that the train is currently occupying. It does not show the next signal aspect, but the next signal can be inferred by the cab signal currently shown.

Automatic Train Stop, or ATS, systems provide wayside inductors that, when activated, signal the train to apply its brakes. Automatic Train Control, or ATC, adds in-cab enforcement to these and will apply the brakes if a dangerous situation arises, such as when the next signal is displaying a stop indication but the engineer has not begun slowing the train. ATC is required on all U.S. rail lines that operate at more than 79 miles per hour.

A further enhancement designed to work in both signaled and dark territory is Positive Train Control, or PTC. This system is an overlay on the conventional methods of operation but also uses satellite-based tracking and computerized radio communication to verify the authority given to the train, current location of the train, the status of the next signal (if any), the position of switches (which will be equipped with a sensor and radio transmitter), and the location of any oncoming trains. As in ATC, if a dangerous situation arises, the system will apply the brakes.<ref>Positive Train Control: Intelligent Railroad Systems, Federal Railroad Administration, retrieved August 16, 2006</ref><ref>Most Wanted List of Transportation Safety Improvements, National Transportation Safety Board, retrieved August 16, 2006</ref><ref>BNSF starts positive train control trial - North American Viewpoint, by William Vantuono, International Railway Journal, March, 2004, retrieved August 16, 2006</ref>

[edit] References

<references/>

• Railroad's traffic control systems by Frank W. Brian, Trains May 1, 2006, retrieved August 16, 2006
• General Code of Operating Rules, Fifth Edition. Copyright 2005 General Code of Operating Rules Committee (modified by BNSF Railway Company)

[edit] See also

• Automatic train protection system
• List of rail diagrams
• Railway signal
• Railroad switch
• Traffic sign
• Train speed optimization

Country-specific signalling

• Australian railway signalling
• German railway signalling
• Norwegian railway signalling
• Swedish railway signalling
• UK railway signalling
• North American railway signaling

[edit] External links

• The European Railway Signalling Server
• Docklands Light Railway signalling system
• Railroad Signalling: North American Signaling and Safety
• Railroad Signaling and Communications - photos and info
• RailServe.com Signals & Communications - 100+ links
• Information on British "Absolute Block" signalling
• Railways: History, Signalling, Engineering [1] [2] [3] [4] and [5]
• Railway Signalling and Operations FAQ
• North American Signaling
• Safeworking systems in Victoria (Australia)es:Bloqueo

fr:Signalisation ferroviaire he:סימנור pl:Sygnalizacja kolejowa ru:Железнодорожная светофорная сигнализация sk:Zabezpečovacie zariadenie